Optical signal emission systems, or optical injectors, are known, for which the emission is at the same time stable, single-mode and single-frequency, for which the emission power is limited to a few tens of milliwatts.
This limitation is particularly strong around the wavelength of 1.55 μm, which is often employed for its ocular safety.
The applications which require a greater power, within a power range greater than an optical watt, have to use at least two amplification stages. A first amplification stage, or preamplifier, makes it possible to reach the power range required for an output signal which is then amplified by a second amplification stage so as to reach optical powers greater than a watt. These amplification stages each require a specific interface: a pump source, a cooler, a mixer, a splitter and possibly a residual pump recycler.
The presence of these two amplification stages greatly increases the cost and the size of these emission systems and involves a large number of connections between components, which reduces the reliability and complicates maintenance.
The high-gain amplification of a single-mode signal is generally produced in guided optics by two well known embodiments, which both use laser diodes as pumps, thus exploiting their compactness and their reliability.
The first known embodiment consists of the use of a fibred single-mode pump laser diode. This embodiment allows for an effective amplification because its emission is matched to the emission of the amplifying medium. On the other hand, the pump power for an emitter is limited around the optical watt, limiting the length of the amplifying material and therefore the overall gain of the amplifier. In order to obtain a high-gain amplification, several of these pump laser diodes must be used, which reduces the compactness and increases the cost, in terms of purchase price of the pumps and the electrical consumption needed to cool each of the pumps.
The second known embodiment involves using one or more wide-strip laser diodes and matching the amplifying medium to the multimode emission of the pump laser diode. There are double-core optical fibres in which only a narrow central cylinder is active. Thus, it is possible to generate a single-mode signal around this region while the pump wave is guided in the wide cylindrical part surrounding the narrow active central cylinder. However, the weak interaction between the multimode pump and the single-mode signal of such an embodiment makes it necessary to have a long length of optical fibre in order to obtain a significant overall gain.
Moreover, the optical amplifiers require a mixer at the input and a splitter at the output between the pump and the signal. Produced more often than not by fusion-drawing between two optical fibres, these components reduce the compactness of the amplification modules.
Furthermore, it is sometimes proposed to produce the pump on each side of the amplifying medium in order to obtain a uniform pumping, limiting the possible stray effects due to an excessively high power density in the active medium. However, the residual pump power which leaves on either side of the amplifying medium may be coupled in each of the laser diodes. Their emission is disturbed, the emitted power and the wavelength may then substantially vary over time. These disturbances increase the additional noise of the amplifier. It may then prove necessary to use a costly fibred optical isolator at the output of each pump laser diode.
Also, the known embodiments for high-gain amplification lack compactness. This limitation is due to the pumps available. In the case of single-mode laser diodes, a number of separate emitters have to be used, and in the case of multimode laser diodes, a length of several meters of amplifying medium must be employed. Furthermore, the addition of mixer, splitter and isolator integrated on optical fibre contributes to the lack of compactness of the system.